The dense, low-resistance and reliable thick-film conductor circuit must have a short wiring pitch and incorporate a conductor having a large cross sectional area, that is, having a large height and satisfactory chemical stability, for example, acid resistance and corrosion resistance.
For example, the number of pins in each of IC and LSI (Large Scale Integrated Circuit), the performance of each of which has been improved in recent years, has been enlarged to meet a requirement for realizing improved performance. Moreover, the pitch of the pins has gradually been shortened. Liquid crystal display units have been required to have precise structures. Under the foregoing circumstances, the number of display scanning lines has significantly be enlarged. Moreover, intervals of electric wires have been excessively shortened to easily mount ICs for controlling display. Under the above-mentioned technological circumstances, there arises a requirement for a dense and low-resistance element for connecting different-pitch wiring elements to each other such that elements having considerably different wiring pitches, for example, an IC or an LSI having pins configured at small pitches and a mother board (having electric wires configured at long pitches owning to a manufacturing requirement) are connected to each other. Note that an element of the foregoing type is defined as an "interposer" in a document (Nikkei Electronics, 1995. 1. 16 (No. 626) pp. 79 to 86). In this specification, the foregoing element is defined as a "different=pitch-element connecting member". As for performance which must be satisfied by the different-pitch-element connecting member, a report has been published in 6-th Microelectronic Symposium, pp. 43 to 46 about a fact that signal pulses are attenuated in inverse proportion to the resistance of the wiring when the CPU is operated at clock speed for a high speed operation. If the resistance of the wiring for connecting a control IC for a precise liquid crystal display device and a liquid crystal substrate is raised, the S/N ratio is lowered and thus the image becomes instable. As a result, the different-pitch-element connecting member must have a low-resistance circuit, short wiring pitches and satisfactory reliability.
As a means for improving the chemical stability of a conductor, plating of zinc, which has a base standard potential as compared with that of copper, on the surface of copper has been performed to prevent corrosion of the copper. However, the zinc plating method encounters a fact that the physical properties of a formed film are adversely affected by the plating conditions, that is, the composition of plating solution, the density of contained impurities, temperatures during a plating process and the density of electric currents. Therefore, the plating solution and process must delicately be controlled. A zinc film must have a large thickness to obtain the corrosion resistance. If the thickness and ratio of zinc having inferior electric conductivity to that of copper are enlarged, there arises a problem in that the electric conductivity deteriorates. Since a process using high frequencies encounters a problem in that the resistance of the surface of the conductor is raised, the surfaces of the conductor is sometimes plated with a noble metal, such as gold. However, a means cannot be obtained which is capable of reducing the thickness of a zinc film, maintaining the electric conductivity and preventing corrosion and oxidation of a copper pattern, which is an object of the present invention and which is applicable to a dense conductor circuit having a large surface area with respect to the cross sectional area of the conductor.
As a method of manufacturing a dense and thick-film conductor circuit, a method is known in which a copper-applied substrate, photolithography and etching are combined with one another. However, a thick-film conductor circuit obtainable from the above-mentioned method encounters difficulty to prevent a side etching phenomenon which takes place when copper foil is etched. If the density is raised, the reliability of the conductor deteriorates. In proportion to the height of the conductor, the above-mentioned phenomenon becomes critical. Therefore, the thickness of copper foil of usual copper-applied substrates is about 18 .mu.m to 35 .mu.m. Also the wiring density is lower than 20 lines/mm.
Another method of manufacturing a thick-film conductor circuit, methods have been disclosed in Japanese Patent Laid-Open No. 52-137666 and Japanese Patent Laid-Open No. 57-162489, in which a resin pattern is filled with a thick-film paste material. Another method is disclosed in Japanese Patent Laid-Open No. 55-41729, in which thick-film paste and a positive-type resist are mixed with each other, after which exposure and development are performed. However, the above-mentioned methods cause a thick-film conductor circuit to be obtained in which voids are formed in the conductor, a defect takes place owning to short circuit between conductors and the characteristics of the circuit deteriorate because of rise in the resistance of the conductor.
In Japanese Patent Laid-Open No. 59-198792, a method of manufacturing a thick-film conductor circuit has been disclosed which has the steps of: repeating lamination and exposure of photosensitive resin such that a conductive substrate is employed as a base, and performing development and electric plating. Although expectation can be made that the above-mentioned method will enable a resist pattern having a relatively high aspect ratio to be obtained and a low-resistance conductor circuit to be manufactured, a complicated process must be performed. What is worse, the deviation which occurs when the lamination and exposure are performed causes a defect to be introduced and short circuit to occur because of a defective resin change. Thus, there arises a multiplicity of problems in that the reliability of the circuit is unsatisfactory.
In Japanese Patent Laid-Open No. 56-94690 and Japanese Patent Laid-Open No. 60-161605, methods each of is capable of forming a fine pattern circuit have been disclosed, in each of which a conductive substrate and photolithography are combined with each other after which anisotropic electrolytic plating is performed. The foregoing method controls the design of the conductor by the plating start width. What is worse, the conductor has a spherical shape. Therefore, the above-mentioned method is unsatisfactory as a method of manufacturing a dense and low-resistance pattern conductor.
Moreover, the applicant of the present invention has disclosed, in Japanese Patent Laid-Open No. 6-260740, a method of processing a substrate for the purpose of obtaining a resist pattern having a high aspect ratio by improving the adhesiveness between photosensitive resin and conductive substrate. However, a thick-film conductor circuit obtainable from the above-mentioned method cannot include a predetermined layer containing zinc atoms in a surface layer of the conductor. Thus, there arises a somewhat problem in that satisfactory oxidation resistance cannot be obtained.
In contrast to the above-mentioned printed circuit industrial field, significant developments have been made in a micro-machine field including a medical field and a sensor technology field. As an example of application of a method of manufacturing a micro-machine to a process for manufacturing a pattern conductor, methods of manufacturing microcoils have been suggested in Microelectronic Engineering 4 (1986), 35 and Proc. IEEE Solid-State Sensor & Actuator Workshop June (1988) P1, in each of which X-ray lithography (a LIGA process) is performed such that PMMA resist and synchrotron radiated light are combined with each other. The above-mentioned method enables a significantly dense, low-resistance and thick-film pattern conductor to be manufactured. However, a costly X-ray source inhibits cost reduction. Moreover, the sizes of substrates which can be employed are limited. Thus, there arises a problem in that the industrial manufacturing process cannot easily be performed.
In place of the LIGA process, fine pattern forming techniques each using photosensitive polyimide have been disclosed in Transducers '93, Yokohama, June (1993), 7th Int. Con. Solid-State Sensor & Actuator and so forth. The above-mentioned techniques are techniques for applying photolithography and plating to the surface of a silicon substrate so as to form a micro-gear or a comb electrode. However, a technique for industrially and less-expensively manufacturing a dense and low-resistance conductor pattern has not been disclosed.
In contrast to the above-mentioned techniques each of which is capable of forming a fine pattern conductor, the applicant of the present invention has disclosed a method of manufacturing a thick-film fine pattern having a high aspect ratio in Japanese Patent Publication No. 5-81897. Moreover, the applicant has disclosed liquid photosensitive resin having improved adhesiveness with a conductive substrate in Japanese Patent Laid-Open No. 6-283830. However, the photosensitive resin composition as disclosed above has a problem in that residues are somewhat left in non-exposed portions of the photosensitive resin composition in the lithography process.
The difference between a photosensitive resin composition according to the present invention and the photosensitive resin composition disclosed in Japanese Patent Publication No. 5-81897 will now be described. The characteristic of the photosensitive resin composition according to the present invention lies in that the light transmittance of the photosensitive resin composition is controlled. In contrast to the photosensitive resin composition disclosed in Japanese Patent Publication No. 5-81897 in which only a polymerization initiator controls exposure, the method according to the present invention has a structure that both of a polymerization initiator and a photoabsorber are used to control the light transmittance.
For example, a photosensitive resin composition having a light transmittance of 70% enables a resist pattern to be manufactured in either of a case where only the polymerization initiator is used or a case where both of the polymerization initiator and the photoabsorber are used. However, when only the polymerization initiator is used in a large quantity, the resolution and development characteristic deteriorate excessively. If a dense pattern of 100 .mu.m or finer is formed, there arises problems of residues and fogging. If both of the polymerization initiator and the photoabsorber are used, photopolymerization reactions which take place when exposure is performed can be controlled. Simultaneously, light transmission can be adjusted. Therefore, a very dense pattern of 100 .mu.m or finer can be formed. When the combination of the photoabsorber and the photoabsorber to obtain effects of preventing residues in the development process and fogging is employed, the photoabsorber must prevent light transmittance in a quantity of 15% or greater.
FIG. 5 shows the relationship between thicknesses and light transmittance of the photosensitive resin composition according to the present invention and the photosensitive resin composition disclosed in the embodiment of Japanese Patent Publication No. 5-81897. As can be understood from FIG. 5, the photosensitive resin composition disclosed in Japanese Patent Publication No. 5-81897 having light transmittance which is higher than that of the photosensitive resin composition according to the present invention enables a resist pattern having a large thickness to be formed. However, a dense pattern of 100 .mu.m or finer cannot be obtained. The photosensitive resin composition disclosed in Japanese Patent Publication No. 5-81897 encounters reduction in the light transmittance of 75% or lower if the thickness of the formed film is enlarged. Also in this case, non-exposed portions encounter residues and fogging.
The present invention and Japanese Patent Publication No. 5-81897 are different from each other in prepolymer. That is, the present invention has a structure that a compound having components expressed by the following chemical formulas (1) and (2) is employed in addition to dicarboxylic acid which is a raw material of the prepolymer. Thus, a photosensitive resin composition simultaneously having unexpected high resolution, a satisfactory development characteristic and plating resistance was found. Moreover, when a compound having a component expressed by the following chemical formula (3) is blended at a specific ratio, the foregoing effect can furthermore be improved. ##STR1##
where R.sub.1 and R.sub.2 are each --COOH or --CH.sub.2 COOH and R.sub.3 and R.sub.4 are each --H or --CH.sub.3. EQU HOOC--(CH.sub.2)n-COOH (n=3 to 8) Chemical Formula (3)
Since the photosensitive resin composition according to the present invention contains the relatively reactive prepolymer, the reservation stability for a long time is inferior to that of another prepolymer. Therefore, employment of the foregoing photosensitive resin composition has not been considered. However, a fact that the foregoing resin is significantly advantageous resin as photosensitive resin for forming a thick-film conductor circuit was found.